This invention relates to the field of electronic oscillator circuits and more particularly to low power, low voltage crystal oscillator circuits that find application in battery operated personal paging receivers.
In the design of selective call or paging receivers it is desirable to prolong the operating time between battery charges or replacement. As the physical size of paging receivers has been reduced over the years, the size and electrical capacity of their batteries have also been reduced, potentially causing a corresponding reduction in the paging receiver's operating time. To compensate for the reduced battery capacity, development work has been directed towards reducing the power drain of all circuits in the paging receiver. This work has lead to the development of both low power circuit designs and to the development of sophisticated switching methods, or "battery saver techniques", whereby portions of the paging receiver are switched ON only for brief intervals and only when they are required to be on, as determined by the selective call coding protocal.
A paging receiver is usually powered from a one cell battery having a voltage in the range of 1.1 to 1.5 volts. Ideally, the paging receiver circuits are powered directly from the battery, however, some circuits will not operate at these low voltages and it becomes necessary to add a DC-DC converter to step-up the voltage. Because DC-DC converters are less than 100% efficient and they require larger and more expensive components as their load increases, it is desirable to operate as few circuits as possible from the DC-DC converter.
Paging receivers typically contain a selective signalling decoder to decode the information transmitted to the paging receiver. The decoder uses a crystal oscillator to provide accurate time base signals for its operation. CMOS microcomputers, which require the stepped-up voltage, are commonly used as decoders.
It is desirable that the paging receiver immediately generate a "turn on alert signal" upon the activation of the ON switch to inform the user that the battery is sufficiently charged and that the paging receiver is operating properly. The generation of this alert signal requires that the decoder oscillator start up as quickly as possible, for even a delay of one second between the activation of the ON switch and the generation of the alert signal has been demonstrated to be unacceptable to the users of paging receivers.
It is well understood in the art that the start up time of a crystal oscillator circuit is inversely proportional to both the bandwidth of the crystal and the gain of the oscillator circuit at the crystal frequency. The high Q or quality factor associated with quartz crystals results in very narrow crystal bandwidth and, therefore, potentially long start up times. Therefore, it is necessary to provide high circuit gain to achieve reasonable start up times. However, unless the circuit gain is reasonably predictable and stable, too high a circuit gain may cause the circuit to oscillate in an uncontrolled mode, or to oscillate at a spurious frequency associated with an undesired crystal response.
Presently, low power, low voltage oscillator circuits have been fabricated using CMOS integrated circuit technology, but these circuits have suffered from a number of deficiencies including highly variable power drain characteristics, low and inconsistent amplifier gains that cause unpredictable oscillator start up times, and the inability to resume oscillation under all supply voltage conditions when the power source is interrupted. These operational deficiencies are caused by the normal variations in the characteristics of transistors fabricated with conventional CMOS integrated circuit technology. In particular, the gain of CMOS amplifiers is inherently low when they are operated at low power supply voltages. This gain reduction is especially drastic when the power supply voltage is reduced to the lower end of the normal voltage range of a one cell battery, typically 1.1 volts.
To overcome these problems, many CMOS oscillators are intentionally left ON at all times, even when the paging receiver is turned OFF. This eliminates the variable start up time problem and only requires the oscillator to start when a new battery, with high terminal voltage, is placed in the paging receiver.